The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system.
As illustrated in FIG. 1, an E-UTRAN typically comprises user equipments (UE) 150a, 150b wirelessly connected to radio base stations (RBS) 100a, 100b, 100c commonly referred to as eNodeBs (eNB). In such a wireless communication system, it is desirable to reuse as much of the radio resources in each cell 110a, 110b, 110c as possible to achieve good spectral efficiency. However, whenever a resource is reused this may lead to interference (intra-cell or inter-cell interference) between the links utilizing this particular resource.
Various approaches to manage the interference are known. One approach relies on actively selecting which UEs that can access a particular resource based on channel state information for these UEs. Of all possible UEs, those who interfere the least with each other may be scheduled jointly. Another approach is to use information about eNB and UE antenna polarization when allocating radio resources to the UEs, so called polarization-based interference management, as the isolation is good (i.e. the interference is low) between orthogonally polarized transmissions.
The benefit of using antenna polarization information when scheduling radio resources to UEs is greatest when the transmissions to and from UEs are primarily either vertically polarized (VP) or horizontally polarized (HP). The reason for this is that these polarizations are well preserved in the wireless propagation channel, even in heavily shadowed situations. A transmitted vertically polarized wave will thus keep its vertical polarization throughout the propagation to a receiving side with a very limited cross-polarization scattering, and the corresponding is true for a horizontally polarized wave. In contrast, polarizations that contain elements of both VP and HP are not as well preserved and will therefore be less useful.
Due to the limited amount of cross-polarization scattering in the radio channel, the vertical-to-vertical and horizontal-to-horizontal polarization combinations (transmitting antenna and receiving antenna polarization combination) provide an equal received signal power on average, while the cross-polarized combinations (vertical-to-horizontal or vice versa) typically provide 5-15 dB weaker received signal power. A signal transmitted by an eNB with a VP transmission antenna will thus be received as a stronger signal in a UE with a VP receiving antenna (i.e. a vertical-to-vertical polarization combination) than in a HP receiving antenna (vertical-to-horizontal polarization combination).
The basic idea of polarization-based interference management is in the following described with reference to FIG. 2a. A UE 150b with a VP antenna would benefit from being scheduled to transmit to or receive from eNBs 100b, 100c utilizing a transmission/receiving mode generating a VP, thus providing a VP-to-VP combination channel 220, jointly with a second UE 150a that has a HP antenna and that is scheduled to eNBs 100c, 100a utilizing a transmission/receiving mode generating a HP and thus providing a HP-to-HP combination channel 210. The interference suppression between the two UEs 150a, 150b will be an additional 5-15 dB compared to if the two UEs would both have been using the same polarization combination (i.e. non-orthogonal polarization).
There are some properties that will enable the system described in FIG. 2a to provide the best possible polarization-based interference suppression. One first property is the ability of the eNBs to distinguish VP and HP from other polarizations in uplink, i.e. to assure that the receiving antennas of the RBSs generate a VP or HP, so that they can determine the UEs polarizations through uplink power measurements. A second property is the ability of the eNBs to transmit these two polarizations that are well preserved on the downlink, i.e. to assure that the transmitting antennas of the RBSs generate a VP or HP also in downlink. A third property is the common understanding of which polarization is VP and which is HP, both within an eNB between the receiving and the transmitting side, and between two or more eNBs.
One straight forward solution to assure the first and second properties described above, is to use VP/HP-polarized antenna configurations, and to apply the output power to one antenna at a time. However, there are several reasons why slant +45/−45 polarized antennas are preferred to VP/HP polarized antennas. One reason is that slant +45/−45 polarized antenna configurations provides symmetry of the radiation patterns. Another reason is that slant +45/−45 polarized antennas generate polarization orthogonality away from bore sight, i.e. in all directions. Yet another reason has to do with power amplifier (PA) utilization. As there is one PA for each antenna, it is preferable to transmit a signal using both antennas, as the output power then can be doubled compared to if only one antenna and one PA is used. With a slant +45/−45 polarized antenna configuration, a VP or HP transmission would use both antennas and both PAs. With a VP/HP antenna configuration, a VP or HP transmission would use only one antenna and PA, thus leading to less output power. The drawbacks of VP/HP polarized antenna configurations has resulted in that the overwhelming majority of existing sites with dual-polarized antennas utilize slant +45/−45 polarized antennas.
However, when slant +45/−45 polarized antennas are used, the transmitters and receivers need to be phase coherent to ensure that VP/HP polarized signals can be distinguished. The coherency should be relative, which can be achieved using calibration circuitry, such that the phase offset is constant over time and frequency. Furthermore, the phase offsets of the antennas required to achieve absolute VP or HP transmission or reception needs to be known. One solution to find the phase offsets required is to introduce external calibration equipment to detect and adjust the absolute polarization by tuning the phase offsets of the two cross-polarized antennas. This is however a costly solution as external equipment is needed.
Still another solution is to adjust the polarization of the transmitting antenna configuration based on a quality indicator, such as the rank indicator or the throughput, of the communication between the eNB and the UEs. The basic idea of such a solution, is that an absolute VP/HP polarization of the transmissions, will result in an optimal quality of the communication due to the beneficial characteristics of such polarizations described above. A quality indicator may thus be used to determine if an adjustment of the antenna configuration polarization, through a change of the phase offset between the two cross-polarized antennas, is improving the quality of the communication or not. By iterating the steps of checking the quality indicator and of adjusting the polarization until an optimal value of the quality indicator is reached, it is possible to determine what phase offsets that generate a VP or a HP. A calibration of the absolute polarization state is thus achieved, but an ambiguity regarding the interpretation of VP and HP will still remain since it is not possible to find out what phase offset that generates VP and what phase offset that generates HP, which is a prerequisite for using polarization-based interference management as described with reference to FIG. 2a above. Furthermore, the phase offsets of the receiving antenna configuration are not established with the above described methods.
Using VP/HP-polarized antenna configurations would probably not either solve the problem of not knowing what offset that would generate VP and what offset that would generate HP, as there is normally no control at site installation that the VP antenna is always connected to the first antenna port and the HP antenna to the second antenna port. Applying all power to the antenna connected to the first antenna port will thus generate either a VP or a HP, but it is not known which one without having to take special care to the antenna-to-antenna port connection during site installation. Thus, also in this case, the ambiguity regarding the interpretation of VP and HP is a problem.
Due to the ambiguity problem, the eNBs will have to assume which amplitude and/or phase offsets that will result in VP and HP respectively, both on uplink and on downlink. This assumption creates problems when using polarization-based interference management, as the third property mentioned above regarding the common understanding of which polarization is VP and which is HP between the receiving and the transmitting side of an eNB or between two or more eNBs, will not be fulfilled. How this affects the interference suppression is described in the following with reference to FIG. 2b. One of the three eNBs 100c has assumed the opposite definition of VP/HP for the transmitting antenna compared to the other two 100a, 100b. If a scheduler orders simultaneous transmission from eNBs 100c and 100a to a first UE 150a with HP, and from eNBs 100c and 100b to a second UE 150b with VP, then eNB 100c will transmit with opposite polarizations compared to the other eNBs 100a, 100b, leading to severe interference at each UE 150a 150b. 